1. Field of the Invention
The present invention relates to image processing circuits, imaging circuits, and electronic devices, and particularly to an image processing circuit, an imaging circuit, and an electronic device for use in processing a captured image.
2. Description of the Related Art
In recent years, digital still cameras and digital video cameras have become widespread, and portable terminals have incorporated camera functions, so that imaging circuits equipped with a solid-state image pickup device have grown in demand.
FIG. 16 shows a part of an imaging circuit using a solid-state image pickup device.
The imaging circuit includes a solid-state image pickup device 80 and an image processing circuit 90.
The image processing circuit 90 includes a switch sw91 for connecting or disconnecting the solid-state image pickup device 80, a differential amplifier (comparator) 91, a ramp signal supply source 91a for supplying a ramp signal, a capacitor C91 connected to a non-inverting input terminal of the differential amplifier 91, a capacitor C92 connected to an inverting input terminal, a switch sw92 for connecting or disconnecting the output terminal and the inverting input terminal of the differential amplifier 91, an increment counter 92 for counting up in accordance with the output value of the differential amplifier 91, an AD conversion clock 93 for supplying an operation clock signal to the increment counter 92, and an ADC control circuit 94 for controlling AD conversion by supplying a reset signal to the increment counter 92 or the like.
The conventional operation of the imaging circuit will be described next.
FIG. 17 is a timing chart illustrating the conventional operation of the imaging circuit.
In FIG. 17, a connection node between the non-inverting input terminal of the differential amplifier 91 and the capacitor C91 is denoted as n91; a connection node between the inverting input terminal and the capacitor C92 is denoted as n92; and a connection node between the output terminal of the differential amplifier 91 and the increment counter 92 is denoted as n93.
First, initial reading is carried out. To be more specific, noise reading (N Read) is performed first, and the capacitor C92, connected to the inverting input terminal, holds the voltage equivalent to the noise voltage (T90-to-T91 time segment).
Then, signal-plus-noise reading (S+N Read) is performed, and the signal-plus-noise voltage is input to the non-inverting input terminal of the differential amplifier 91 (T91-to-T92 time segment). These operations give the noise voltage to the inverting input terminal and the signal-plus-noise voltage to the non-inverting input terminal, bringing the potential difference between the two input terminals to the signal voltage.
Next, the switch sw91 is turned off, and the signal-plus-noise voltage is held at the non-inverting input terminal. This operation ends the initial reading.
The AD conversion clock 93 is activated (in a shaded portion in the figure), and the potential of the non-inverting input terminal coupled to a capacitor is driven by a ramp signal, thereby ramping up the signal-plus-noise voltage in the direction of the noise voltage. The counter counts up until the output of the differential amplifier is inverted and stops when the output is inverted, thereby performing AD conversion (T92-to-T93 time segment).
In the conventional operation, an encoded value (digital output value) output from the image processing circuit 90 equals the counter value counted by the increment counter 92.
It is increasingly required that the imaging circuit generate an imaging signal with reduced noise. One known method to reduce the effect of noise is an imaging circuit having a function to cancel out noise generated in a pixel portion of the image pickup device (refer to Japanese Unexamined Patent Application Publication No. 2005-136540).
In the conventional operation, however, white noise caused by thermal noise or the like causes images with a low signal-to-noise ratio, such as a dark image, to become coarse, like snow.
White noise caused by thermal noise or the like is added to a voltage value encoded in AD conversion. If this white noise is greater than the potential difference per bit (LSB) in AD conversion, the noise component remains in the encoded value. Noise varies with time, causing each pixel having the same brightness, not varying with time, to flicker frame to frame, causing a dark image to become coarse like snow. Recent demands for high resolution and high picture quality create a trend toward a reduced signal per pixel and an increased resolution, worsening the S-N ratio. This problem has become apparent especially in a dark image, having a low S-N ratio.